Mouse embryo cells are considered as a nonpermissive host for the growth of SV40. The virus acts primarily as endogenous mitogen by stimulating host DNA replication. Experiments were carried out to study the expression of SV40 during abortive infection which is the initial step of transformation. Primary transcripts of wild type SV40 and a temperature sensitive mutant obtained early after infection of mouse embryo cells were analyzed and compared to viral RNA transcripts from infected and transformed cells. It has been shown by others that these cells do not express late gene products. The block for the synthesis of the SV40 late proteins was also studied. Murine embryonal carcinoma cells are the stem cells of teratocarcinomas. These cells infected with SV40 do not synthesize authentic SV40 proteins. Experiments were designed to study the nature of the block which prevents the synthesis of viral proteins. Experiments include infection of these cells with SV40 and analysis of viral RNA transcripts. Sizing of the transcripts should determine whether the RNA molecules are processed correctly. The RNA molecules will be tested on in vitro protein synthesizing system for messenger RNA activity.